No abstract

The gradients of root exudates and of persistent organic pollutants, including polycyclic aromatic hydrocarbons (PAHs), in rhizosphere soil in proximity to the root surface are still not well elucidated. In this work, a greenhouse experiment was conducted to investigate the distribution gradients of root exudates and phenanthrene and pyrene, as selected PAHs, in rhizosphere soil close to (0-8 mm) the root surface of ryegrass [Ldium multiflorum Lam.) as the host plant. Rhizosphere soil from the root surface (0-8 mm) was divided into three layers: the rhizoplane and strongly and loosely adhering soil. Root exudates were characterized as soluble organic C, organic acids, and total soluble sugars. In PAH-spiked rhizosphere soils, the concentration of root exudates decreased with distance from the roots. In a sterilized treatment, the amounts of root exudates in the three rhizosphere layers were higher than when the soil was not sterilized, indicating that microbial consumption contributed significantly to the loss of root exudates in the rhizosphere. Tlie residual concentrations of phenanthrene and pyrene clearly increased in die order of rhizoplane to loosely adhering soil after 40 to 50 d, which was significantly and negatively correlated with the amount of root exudates in the rhizosphere. In total, 87 to 97% of phenanthrene and 69 to 79% of pyrene dissipated in all three layers of rhizosphere soil after 40 d, and >99% of phenanthrene and 93% of pyrene dissipated after 50 d. The degradation ratio of PAHs decreased in rhizosphere soils with distance from the roots.Abbreviations: OA, organic acid; PAH, polycyclic aromatic hydrocarbon: SOC, soluble organic carbon: TSS,) total soluble sugar.

No abstract

Soybean [Glycine max (L.) Merr.] stalk-based biochar was prepared using oxygen-limited pyrolysis. We evaluated phenanthrene (PHE) and Hg(II) sorption, from single and binary component solutions, onto prepared biochar. We found that the prepared biochar efficiently removed PHE and Hg(II) from aqueous solutions. The isotherms for PHE and Hg(II) sorption could be described using linear and Tóth models, respectively, both with high regression coefficients (R(2) > 0.995). When PHE and Hg(II) coexisted in an aqueous solution, we observed direct competitive sorption, each one suppressing another. Our results provide insight into the recycling of agricultural residues, and also a new application for removal of polycyclic aromatic hydrocarbons and heavy metals from contaminated water utilizing biochar from agricultural residue.

The impact of low‐molecular‐weight organic acids (LMWOAs) on the sorption–desorption of phenanthrene, used as a representative of polycyclic aromatic hydrocarbons, was investigated with a laboratory batch technique. Experiments were conducted with three types of soil samples containing various organic components: citric, oxalic, and malic acids. The sorption of phenanthrene by soils could be well described by a linear‐type model irrespective of the addition of LMWOAs. The simulated distribution constant (Kd) and C‐normalized distribution constant (Koc) for phenanthrene sorption decreased significantly with the addition of LMWOAs to the soils (P < 0.05). In contrast, the desorption amounts of phenanthrene were clearly enhanced by the addition of LMWOAs, and the impact of citric acid on the sorption–desorption of phenanthrene was generally more significant than those of oxalic or malic acids, probably due to their chemical structures and properties. Compared with the baseline soil samples, the desorption amounts of phenanthrene in the presence of LMWOAs were significantly decreased in soils after 60 d of cultivation (P < 0.05), whereby the level of the fractional soil organic C content (foc, %) increased proportionally with the inhibition strength of phenanthrene desorption in the presence of LMWOAs. Based on these observations, we suggest that the availability of polycyclic aromatic hydrocarbons can be enhanced with suitable types and concentrations of LMWOAs.

Persistent organic pollutants (POPs) accumulation in plants poses a risk to human health. POPs can be transported to plants through soil and air by abiotic or biotic processes. Their transport mechanisms in plants include symplastic and apoplastic processes, depending on the POP physicochemical properties and the vegetation species and growth stage. POPs with higher log octanol–water partition coefficient (Kow) values were absorbed more easily than those with lower logKow values. Specific endophytic bacteria showed increased expression of specific genes, more efficient degradation of organic contaminants, and the ability to degrade and reduce organic contaminants in host‐plant tissues. Several POP‐degrading endophytic bacteria were isolated from plants grown in POP‐contaminated soils and then inoculated successfully into plants. They expressed the genes encoding enzymes in the POP degradation pathways and reduced phytotoxicity and the amount of POP present in plant tissues. Thus, plant colonization by endophytic bacteria can be applied to degrading and reducing the concentration of POPs in plants.

Endophytic bacteria can promote plant growth, induce plant defence mechanisms, and increase plant resistance to organic contaminants. The aims of the present study were to isolate highly PAH-degrading endophytic bacteria from plants growing at PAH-contaminated sites and to evaluate the capabilities of these bacteria to degrade polycyclic aromatic hydrocarbons (PAHs) in vitro, which will be beneficial for re-colonizing target plants and reducing plant PAH residues through the inoculation of plants with endophytic bacteria. Two endophytic bacterial strains P1 (Stenotrophomonas sp.) and P3 (Pseudomonas sp.), which degraded more than 90% of phenanthrene (PHE) within 7 days, were isolated from Conyza canadensis and Trifolium pretense L., respectively. Both strains could use naphthalene (NAP), PHE, fluorene (FLR), pyrene (PYR), and benzo(a)pyrene (B(a)P) as the sole sources of carbon and energy. Moreover, these bacteria reduced the contamination of mixed PAHs at high levels after inoculation for 7 days; strain P1 degraded 98.0% NAP, 83.1% FLR, 87.8% PHE, 14.4% PYR, and 1.6% B(a)P, and strain P3 degraded 95.3% NAP, 87.9% FLR, 90.4% PHE, 6.9% PYR, and negligible B(a)P. Notably, the biodegradation of PAHs could be promoted through additional carbon and nitrogen nutrients; therein, beef extract was suggested as the optimal co-substrate for the degradation of PAHs by these two strains (99.1% PHE was degraded within 7 days). Compared with strain P1, strain P3 has more potential for the use in the removal of PAHs from plant tissues. These results provide a novel perspective in the reduction of plant PAH residues in PAH-contaminated sites through inoculating plants with highly PAH-degrading endophytic bacteria.

Purpose The soil contamination by hydrophobic organic contaminants (HOCs), such as polycyclic aromatic hydrocarbons (PAHs), poses great threats to human health and ecological security and attracts worldwide concerns. The total HOC concentrations overestimate its available fraction to the soil biota. Increased understanding of the availabilities of PAHs in soil environment will have considerable benefits for their risk assessment and be very instructive to food safety and remediation strategies in contaminated sites. However, the availability of PAHs in aging soils and particularly the correlations of the availabilities with their forms in soils have yet to be elucidated. In this work, the availabilities of PAHs in aging soils were evaluated using a sequential mild extraction technique. Materials and methods Four typical zonal soils in China previously free of PAHs were collected from A (0-20 cm) horizon, air-dried, and sieved. Soils were spiked with a solution of phenanthrene and pyrene as representative PAHs in acetone. After the acetone evaporated off, the treated soils were progressively diluted with unspiked soils and sieved again several times to homogenize the soil samples. The forms of PAHs in soils were experimented using microcosms that are similar to those reported in literature. Various treated soils were packed into amber glass microcosms (each with 25 g soil). Three replications were given for each treatment. NaN 3 solution (0.5%) was added to some microcosms in order to get the microbeinhibited treatments. The soil water contents were adjusted to be 20% of soil water-holding capacity. After incubation for 0, 2, 4, 8, 12, and 16 weeks in microcosms with a temperature of 25°C, the soils were sampled. PAHs were then extracted by a sequential mild extraction technique, and their forms and availabilities were determined. Results and discussion The available residual concentrations of phenanthrene and pyrene generally decreased with aging time, and the PAHs were more readily available at the start of the incubation, but their availabilities decreased rapidly with increasing the soil-PAH contact time. In addition, the degradation efficiency of the available PAHs in soils was generally higher for PAHs with low molecular weight. The available residues of PAHs in soils were fractionated into desorbing and non-desorbing fractions. The desorbing fractions were the largest portion of the available PAHs in soils. In addition, the desorbing fractions were the main portion to be readily biodegradable. The non-desorbing fractions of PAHs were less bioavailable and with less possibility to be biodegraded in soils. The dissipation of the desorbing PAH fractions accounted for the dominant contribution to the dissipation of the available fractions of tested PAHs. The formation of bound PAH residues was observed in soils. However, the concentrations of the bound residual PAHs were very low. Comparing with the microbial biodegradation, the transformation of PAHs from available fractions to bound residues was a negligible ...